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Technical ENSO Update

16 January 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-January 2007 conditions indicate that El Niño is still in place but declining. SSTs are currently observed to be approximately 0.5C above average throughout the equatorial Pacific, and remain more than 1.5C in the eastern equatorial Pacific. Deep, or downward, thermocline perturbations exist now only in the far eastern Pacific, as the large subsurface anomalies continue to propagate towards the South American coast and then poleward. In the central equatorial Pacific shallow thermocline anomalies are seen moving eastward. The upwelling Kelvin wave associated with the shallow thermocline anomalies originated in the off-equatorial western Pacific, and has been slightly amplified by large-scale easterly wind anomalies near the dateline in late December.
The shallow thermocline anomalies have been contributing to the weakening of the current El Niño conditions in the subsurface ocean, as has the wind-driven westward advection of cooler water. However, over the last week large-scale westerly wind anomalies, associated with sub-seasonal atmospheric variability, have appeared in the central Pacific. These wind anomalies are likely to stall the recent progression towards ENSO-neutal conditions as they advect more warm water eastward, and deepen the thermocline in the central/eastern equatorial Pacific.

For December 2006, the SSTs in the NINO3.4 region were 1.31 degrees C above average, and for the October-November-December season were 1.13 degrees C above average. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Dec-Jan-Feb and Jan-Feb-Mar seasons is approximately 0.65C and 0.50C away from average, respectively.

Expected Conditions

El Niño conditions exist in the surface and sub-surface tropical Pacific. These include the warm SST anomalies in the eastern equatorial Pacific, westerly wind anomalies in the western Pacific, and a flattening of the equatorial thermocline. Indications that El Niño conditions are waning also exist, which include migration of shallow thermocline anomalies from west Pacific toward the east and an equatorial SOI index that is currently positive (although Tahiti-Darwin SOI has been fluctuating, but it currently negative).

The shallow thermocline sperturbations generated <>in the western Pacific by the westerly wind anomalies of the current El Niño have been converging toward the equator and moving eastward. They were somewhat reinforced by easterly wind anomalies in the central Pacific during late December. Those sub-surface anomalies are eroding the current El Niño event. However, the recent arrival of more westerly wind anomalies are generating deep thermocline anomalies, or more accurately, damping the existing shallow thermocline anomalies in the central Pacific. The westerly anomalies are also likely to contribute to more eastward advection of warm water from the western Pacific.
Thus, the rate at which neutral conditions are restored will depend on several factors, in particular, the constructive or destructive interference of atmospheric weather influencing current and future perturbations to the equatorial upper ocean structure.

Persistence of El Niño conditions remains the most likely outcome through the next few months of 2007. For the JFM season, there is an estimated 85% likelihood for El Niño, approximately 1% probability for La Niña conditions to return, leaving less than 15% probability for ENSO-neutral conditions. By mid-2007, a return to ENSO-neutral conditions becomes the most likely outcome.

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.45 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.

There is little variation among ENSO model forecasts through the first quarter of 2007. Most predictions indicate persistence of weak El Niño conditions. Only a couple of models are forecasting La Niña conditions to occur through the forecast period. For the JFM 2007 season, 18 of 20 models (90%) predict El Niño conditions to continue. At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, only 4 of 13 models (31%) suggest that El Niño conditions will still be in place during the May-Jun-Jul season; 2 of 13 (15%) indicate SSTs exceeding the threshold for La Niña (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will exhibit El Niño conditions until March or April 2007: JFM 2007 (90%) to MAM 2007 (65%), and probabilities for El Niño decline rapidly thereafter. Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. When this method is applied to the current model forecasts, results also indicate a probability of El Niño that is higher than the climatological value (25%)-- approximately 94% from Jan-Feb-Mar 2007 and 62% for MAM 2007. For La Niña the probabilities stay below the climatologically expected 25% throughout the forecast period. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills.

The IRI's probabilistic ENSO forecast takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates a high confidence for the continuation of El Niño conditions March/April 2007 with ENSO-neutral conditions becoming most likely by mid-2007.